This invention relates generally to implantable medical devices and more particularly, it relates to an implantable cardiac defibrillator employing an improved sensing system with non-binary gain changes so as to maintain its output in a desired range.
In recent years, there has been substantial progress made in the research and development of defibrillating devices for providing an effective medical response to various disorders, such as ventricular fibrillation. Research efforts have also been made toward developing improved sensing techniques for reliably monitoring heart activity so as to determine when a defibrillating high energy shock is required.
However the implantable cardiac defibrillators of the prior art used comparatively simple sensing circuits. These prior art sensing circuits would typically include switched capacitor circuits. When it was desired to make a gain change, e.g., by switching in an additional capacitor, the output of the prior art switched capacitor circuits would be increased by a factor of two for each increase in binary gain code. In other words, there would be a doubling of the gains for each binary gain code. If it is desired to maintain an output between a desired range such as normally 65% and 90% of full scale, the prior art gain changing method could result in a hunting or oscillating effect above and below the desired range, thereby destroying the integrity of the sensed ECG heart signals.
It would therefore be desirable to provide an implantable cardiac defibrillator employing an improved sensing system for processing ECG heart signals from the atrial and/or ventricular channels where the gains thereof are changed in a non-binary fashion, thereby avoiding hunting.